Zinc-based alloy coated steel sheets are widely used as building materials and automobile materials because of their good corrosion resistance as structural members. Conventionally, in order to improve the corrosion resistance after non-coated members are welded, the welded members are dipped in a zinc-based alloy bath. This applies the zinc-based alloy to the steel member and the surface of the welded portion, which secures corrosion resistance to the whole welded structure. This method, however, provides low productivity since coating has to be conducted after the welding process. This causes an increase in manufacturing costs since additional facilities, such as a coating bath, are required. In view of this, to manufacture a structural member with good corrosion resistance and with high productivity, a method where zinc-coated steel sheets are welded to form the welded structure has been employed.
JP2000-64061 discloses a zinc-base alloy coated steel sheet in which a zinc-based alloy, such as a Zn—Al—Mg—Si based alloy, is coated. The Zn—Al—Mg—Si based alloy coating has improved corrosion resistance compared to conventional zinc-coated steel sheets.
In the case of manufacturing a structure by welding a zinc-based alloy coated steel sheet, however, corrosion resistance is deteriorated because the coated part of the welded metal portion is evaporated. In view of this, conventionally zinc-based alloy coated steel sheets are first welded using a carbon steel welding material and then the welded portion is coating by brushing or spraying. This additional coating process lowers productivity in the manufacturing of the structural member.
As for welding of stainless steel structures where good corrosion resistance is required, a stainless steel welding material is used to form a welded metal with good corrosion resistance at the joint between stainless steels or stainless steel and carbon steel. However, if a stainless steel welding material is used for welding zinc-base alloy coated steel sheets, cracks occur due to liquid-metal embrittlement. This is because when the zinc-base alloy coated steel is welded, liquid-metal embrittlement cracks form at the welded portion of the stainless steel components due to the melted coating.
The a main cause for liquid-metal embrittlement cracks is thought to be that zinc-based alloy coating components remain melted on the steel sheet. This can break at the crystal grain boundary when the welded metal portion is subjected to tensile stress caused by heat contraction. This is the cause of the brittleness. Therefore, it has been common sense that the zinc-based coating must has be removed in advance when the zinc-coated steel sheets are welded using a stainless steel welding material.
A phenomenon similar to liquid-metal brittleness cracks occurs when different materials, such as a stainless steel sheet and a zinc coated steel sheet, are welded. Therefore, there have been few attempts at welding zinc-coated steel sheets or at welding zinc-coated and stainless steels using a stainless steel-based welding material.
JP09-267177A discloses a manufacturing method of a steel door with good corrosion resistance. In this method, a stainless steel sheet and a zinc-coated steel sheet, both of which are about 2 mm thick, are butt-welded using filler wire with a relatively high Ni content. The Ni content is such to keep the Ni equivalent of the weld metal of stainless steel-based components higher than a predetermined value in order to disperse austenite and inhibit the formation of martensite of poor ductility. This leads to prevention of cracks caused by bending after welding.
In the disclosure of JP09-267177A, there is no description of liquid-metal brittleness cracks (zinc brittleness cracks) that usually occur immediately after the stainless steel-sheet and the zinc-coated steel sheet are butt-welded. In the disclosure of JP09-267177A, a hot-dip zinc-coated steel sheet, of which the melting point of the zinc coating is high, is welded on the condition that the thickness of the steel sheet is 2 mm, and the binding force at the welded portion is weak. This seems to be the reason that liquid-metal embrittlement cracks (zinc brittle cracks) do not occur when the stainless steel sheet and the zinc-coated steel sheet are butt-welded.
However, if a zinc-based alloy coated steel sheet without alloying treatment is butt-welded using the method described in JP09-267177A on the condition that the thickness of the sheet is 3 mm or more and the binding force of the welded portion is high as in a fillet weld, it is supposed to have a phenomenon similar to liquid-metal embrittlement cracks.
The reason why the phenomenon similar to liquid-metal embrittlement cracks occurs easier when the thickness of the sheet to be welded is 3 mm or more and the binding force is higher, is that as the thickness of the sheet increases and/or the binding force increases, the tensile stress, caused by heat contraction of the weld metal, also increases. This causes the zinc-based alloy coating components that remain melted on the surface to break easily at the crystal grain boundary of the weld metal.
When stainless steel sheets are butt-welded, delayed cracks may occur after welding. JP2001-9589A disclosed a method for preventing delayed cracks after welding when a high Cr content stainless steel is welded.
In view of above, it is understood that a method for manufacturing a welded structure, having excellent corrosion resistance in the welded portion, is very difficult when a zinc-based alloy coated steel sheet is welded using a stainless steel-based welding material.